Homotopy theory

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In mathematics, homotopy theory is a systematic study of situations in which maps come with homotopies between them. It originated as a topic in algebraic topology but nowadays it is studied as an independent discipline. Besides algebraic topology, the theory has also been used in other areas of mathematics such as algebraic geometry (e.g., A1 homotopy theory) and category theory (specifically the study of higher categories).



In homotopy theory (as well as algebraic topology), one typically does not work with an arbitrary topological space in order to avoid pathologies in point-set topology. Instead, one assumes a space is a reasonable space; the meaning depends on authors but it can mean that a space is compactly generated Hausdorff space or is a CW complex. (In a sense, “what is a space” is not a settled matter in homotopy theory; cf. #Homotopy hypothesis below.)

Frequently, one works with a space X with some chosen basepoint * in the space; such a space is called a pointed space. A map between pointed spaces are then required to preserve the basepoints. For example, if is the unit interval and 0 is the basepoint, then a map is a path from the basepoint to the point . The adjective “free” is used to indicate freeness of choice of basepoints; for example, a free path would be an arbitrary map that does not necessarily preserve the basepoint (if any). A map between pointed spaces is also often called a based map, to emphasize that it is not a free map.


Let I denote the unit interval. A family of maps indexed by I, is called a homotopy from to if is a map (e.g., it must be a continuous function). When X, Y are pointed spaces, the are required to preserve the basepoints. A homotopy can be shown to be an equivalence relation. Given a pointed space X and an integer , let be the homotopy classes of based maps from a (pointed) n-sphere to X. As it turns out, are groups; in particular, is called the fundamental group of X.

If one prefers to work with a space instead of a pointed space, there is the notion of a fundamental groupoid (and higher variants): by definition, the fundamental groupoid of a space X is the category where the objects are the points of X and the morphisms are paths.

Cofibration and fibration[edit]

A map is called a cofibration if given (1) a map and (2) a homotopy , there exists a homotopy that extends and such that . To some loose sense, it is an analog of the defining diagram of an injective module in abstract algebra. The most basic example is a CW pair ; since many work only with CW complexes, the notion of a cofibration is often implicit.

A fibration in the sense of Serre is the dual notion of a cofibration: that is, a map is a fibration if given (1) a map and (2) a homotopy , there exists a homotopy such that is the given one and . A basic example is a covering map (in fact, a fibration is a generalization of a covering map). If is a principal G-bundle, that is, a space with a free and transitive (topological) group action of a (topological) group, then the projection map is an example of a fibration.

Classifying spaces and homotopy operations[edit]

Given a topological group G, the classifying space for principal G-bundles ("the" up to equivalence) is a space such that, for each space X,

{ principal G-bundle on X } / ~


  • the left-hand side is the set of homotopy classes of maps ,
  • ~ refers isomorphism of bundles, and
  • = is given by pulling-back the distinguished bundle on (called universal bundle) along a map .

Brown's representability theorem guarantees the existence of classifying spaces.

Spectrum and generalized cohomology[edit]

The idea that a classifying space classifies principal bundles can be pushed further. For example, one might try to classify cohomology classes: given an abelian group A (such as ),

where is the Eilenberg–MacLane space. The above equation leads to the notion of a generalized cohomology theory; i.e., a contravariant functor from the category of spaces to the category of abelian groups that satisfies the axioms generalizing ordinary cohomology theory. As it turns out, such a functor may not be representable by a space but it can always be represented by a sequence of (pointed) spaces with structure maps called a spectrum. In other words, to give a generalized cohomology theory is to give a spectrum.

A basic example of a spectrum is a sphere spectrum:

Key theorems[edit]

Obstruction theory and characteristic class[edit]

See also: Characteristic class, Postnikov tower, Whitehead torsion

Localization and completion of a space[edit]

Specific theories[edit]

There are several specific theories

Homotopy hypothesis[edit]

One of the basic questions in the foundations of homotopy theory is the nature of a space. The homotopy hypothesis asks whether a space is something fundamentally algebraic.

Abstract homotopy theory[edit]


Model categories[edit]

Simplicial homotopy theory[edit]

See also[edit]


  • May, J. A Concise Course in Algebraic Topology
  • George William Whitehead (1978). Elements of homotopy theory. Graduate Texts in Mathematics. 61 (3rd ed.). New York-Berlin: Springer-Verlag. pp. xxi+744. ISBN 978-0-387-90336-1. MR 0516508. Retrieved September 6, 2011.
  • Ronald Brown, Topology and groupoids (2006) Booksurge LLC ISBN 1-4196-2722-8.

Further reading[edit]

External links[edit]